Optical scanning apparatus and image forming apparatus using the same

ABSTRACT

A scanning optical system using a short-wavelength light of 500 nm or less uses a reflecting mirror having a higher absolute reflectivity and having reduced wavelength and angle dependences. Divergent ray of light emitted from a semiconductor laser is converted into an approximately parallel light beam by a collimator lens and the diameter of the light flux is reduced by an aperture before travel to a polygon mirror. The light beam from the polygon mirror passes through scanning lenses to form a small spot at any point in the entire scanning area. The semiconductor laser is a gallium nitride semiconductor laser having an oscillation wavelength of 408 nm. The polygon mirror has such a characteristic that, if the complex refractive index N of a metallic film contributing to a reflection characteristic of the reflecting mirror is defined as N(λ)=n(λ)−ik(λ), then k(λ)&gt;{square root}{square root over ((−n(λ) 2 +18n(λ)−1))} is satisfied.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical scanning apparatusused in an image forming apparatus such as a laser printer, a digitalcopying machine, or a multifunction printer.

[0003] 2. Related Background Art

[0004] Ordinarily, optical scanning apparatuses for use in these kindsof image forming apparatus operate in such a manner that a beam of lightfrom a laser light source is deflected by a polygon mirror and travelsthrough an imaging lens system to thereby form an imaging light spot ona surface to be scanned.

[0005] There are many cases where a semiconductor laser or the like isused as a laser light source in a manner described below. Divergent raysof light emitted from a laser light source are converted into anapproximately parallel light beam by a collimator lens and the lightbeam is shaped by an aperture. The shaped light beam enters an imaginglens system after being deflected by a polygon mirror rotating at aconstant angular velocity. It is required that the imaging lens systemhave an fθ characteristic to cause a scanning surface placed at acertain distance from the imaging lens system at a constant distancespeed with the light beam deflected at a constant angular velocity bythe polygon mirror. It is also required that the curvature of field besuitably corrected so that a small light spot can be formed at any pointin the entire scanning area.

[0006] Ordinarily, the imaging lens system is arranged to have a tiltcorrection function for correcting a deviation of the scanning positionin a direction perpendicular to a main scanning direction, i.e., in asub-scanning direction, because the polygon mirror has an error in itsmirror surfaces caused at the time of working for forming the surfaces,and because the rotating shaft of the polygon mirror vibrates.Therefore, the imaging lens system is formed as an anamorphic lenssystem having imaging characteristics differing between the main andsub-scanning directions.

[0007] Conventionally, the imaging lens system has lenses formed from aglass material so as to have a toric surface and a cylindrical surface.Such lenses have an antireflection coating formed thereon by vapordeposition or the like. In recent years, as such lenses, low-costplastic lenses capable of being freely shaped to correct aberrationshave ordinarily been used since working of glass lenses is difficult toperform and the working cost is high.

[0008] Semiconductor lasers conventionally used as light sources areinfrared (780 nm) lasers and visible light (675 nm) lasers. Therefore,polygon mirrors or bending mirrors formed of copper mirrors having ahigh reflectance while having low wavelength dependence and low angledependence have been used.

[0009]FIGS. 8A and 8B show reflectance characteristics of copper itself,and FIGS. 9A and 9B show reflectance characteristics of a conventionalcopper mirror which is formed in such a manner that a copper film isformed on an aluminum base member and alumina (Al₂O₃) and SiO₂ arevapor-deposited on the copper film. As can be understood from thesefigures, the mirror has improved reflection characteristics in theinfrared laser and visible light laser wavelength bands.

[0010] Further, in response to demands for image forming apparatuses ofhigher resolution, the development of optical scanning apparatusescapable of forming a smaller shaped spot is being advanced.

[0011] However, it is apparent from FIGS. 8A, 8B, 9A and 9B that, withdecreasing wavelength, the reflectance of the copper mirror decreasesand its wavelength and angle dependences are also increased. In use ofthe copper mirror with a short-wavelength laser in the conventionalsystem, it is necessary to increase the laser power or to use acollimator lens of a smaller F number in order to maintain apredetermined quantity of light. In such a situation, the load on thelaser itself is considerably heavy and there are cost-increasingfactors, e.g., an increase in the number of collimator lenses forsuitably correcting aberrations.

[0012] In use of a semiconductor laser as a light source in a certainoperational environment, variation in wavelength is inevitable becauseof a temperature-dependent oscillation wavelength characteristic of thelaser. It is, therefore, required that variations in the opticalcharacteristics, i.e., the transmittance, the reflectance, etc., ofoptical components used in the scanning optical system be small in thevicinity of the laser oscillation wavelength. While the copper mirrorhas a good characteristic with respect to infrared laser light andvisible laser light, considerable fluctuation in light quantity, i.e.,image density nonuniformity, results from the wavelength-dependentreflectance of the copper mirror.

[0013] Moreover, because of the angle dependence of the reflectance, theuniformity of image density between the scanning center and scanning endis far from sufficient for formation of a high-quality image.

[0014] In general, optical materials used for forming plastic lenseshave such a transmittance characteristic that, with decreasingwavelength, the transmittance decreases due to absorption in thematerial. High-cost glass lenses have, therefore, been used in opticalscanning apparatuses using short-wavelength light sources.

[0015]FIG. 15 shows a graph showing the transmittance of ordinaryoptical resins. Variation in the transmittance due to internalabsorption in the vicinity of the oscillation wavelength (780 nm) of aninfrared laser or the oscillation wavelength (675 nm) of a visible lightlaser conventionally used as a light source is negligibly small. In thecase of use with a light source of a short wavelength in the vicinity of400 nm, however, the reduction in transmittance due to internalabsorption is not negligible. Also, since the ray passage distance inthe plastic lens changes with respect to the image size, deteriorationin image quality due to light quantity distribution nonuniformity at theposition on the scanned image surface is more considerable than thereduction in absolute quantity of light.

[0016] Also, in use of a semiconductor laser as a light source in acertain operational environment, variation in wavelength is inevitablebecause of a temperature-dependent oscillation wavelength characteristicof the laser. It is, therefore, required that variations in opticalcharacteristics, i.e., the transmittance, the reflectance, etc., ofoptical components used in the scanning optical system be small in thevicinity of the laser oscillation wavelength. In a case where a plasticlens is used in a short wavelength range in the vicinity of 400 nm,there is a problem of image density nonuniformity which, as can beunderstood from FIG. 15, results from variation in the quantity of lighton the scanned surface due to the wavelength dependence of thetransmittance.

SUMMARY OF THE INVENTION

[0017] In view of the above-described problems, an object of the presentinvention is to provide an optical scanning apparatus using a lightsource of a short wavelength not longer than 500 nm, and using areflecting mirror having a high absolute reflectance and smallerwavelength and angle dependences.

[0018] Another object of the present invention is to provide an opticalscanning apparatus in which light quantity distribution nonuniformitydue to absorption in an optical resin is reduced to ensure image densityuniformity.

[0019] In order to solve the above-mentioned problem, according to thepresent invention, there is provided an optical scanning apparatus inwhich a light beam from a light source is deflected and forms an imagingspot on a surface to be scanned, the apparatus comprising: the lightsource having a wavelength of 500 nm or less; and a reflecting mirrorwhich reflects the light beam from the light source, wherein if acomplex refractive index N of a metallic film contributing to areflection characteristic of the reflecting mirror is defined as

N(λ)=n(λ)−ik(λ)

[0020] where n, k>0;

[0021] n(λ) is the real part of the complex refractive index;

i={square root}{square root over (−1)}

[0022] k(λ) is the imaginary part of the complex refractive index(exhaustion factor); and λ is the wavelength, then the reflecting mirrorsatisfies a condition: k(λ)>{square root}{square root over((−n(λ)²+18n(λ)−1))}.

[0023] That is, according to the present invention, in the scanningoptical system in which the wavelength of the light source is 500 nm orless and a reflecting mirror is provided, a suitable metallic materialfor the reflecting mirror is selected to increase the absolutereflectance and to improve the wavelength dependent characteristic andthe angle characteristic of the reflectance.

[0024] In order to solve the above-mentioned problem, according to thepresent invention, there is provided an optical scanning apparatus,comprising a deflection optical system which deflects a light beam froma light source, and a scanning and imaging lens system which forms animaging spot on a surface to be scanned with the light beam from thedeflection optical system, wherein the wavelength of the light source is500 nm or less, and the scanning and imaging lens system has at leastone plastic lens; and if the maximum and the minimum of the total raypassage distance of the plastic lens according to the deflection anglefrom the optical axis is Lmax and Lmin, respectively, thenLmax−Lmin<3·log₁₀0.93/S, and S=log₁₀(1−3.55×10⁸/λ⁴), where λ is thewavelength (nm) of the light beam.

[0025] Also, according to the present invention, there is provided anoptical scanning apparatus, comprising a deflection optical system whichdeflects a light beam from a light source, and a scanning and imaginglens system which forms an imaging spot on a surface to be scanned withthe light beam from the deflection optical system, wherein thewavelength of the light source is 500 nm or less, and the scanning andimaging lens system has at least one plastic lens, and an optical memberhaving a spectral characteristic similar to the inverse of a wavelengthcharacteristic of the transmittance of an optical resin used for theplastic lens.

[0026] That is, according to the present invention, in the scanningoptical system in which the wavelength of the light source is 500 nm orless and at least one plastic lens is provided, a restriction is put onthe selection of the thickness of the plastic lens to reducenonuniformity of distribution of the quantity of light due to internalabsorption in the optical resin, thus guaranteeing image densityuniformity.

[0027] Also, an optical component having a characteristic similar to theinverse of the wavelength characteristic of transmittance of the plasticlens is provided to improve the stability of quantity of light even whenthe wavelength of the light source is changed in the operationalenvironment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In the accompanying drawings:

[0029]FIG. 1 is a schematic cross-sectional view of an essential portionof an optical scanning apparatus which represents a first embodiment ofthe present invention;

[0030]FIGS. 2A and 2B are diagrams showing the spectral reflectance ofan aluminum mirror;

[0031]FIGS. 3A and 3B are diagrams showing the spectral reflectance of apolygon mirror having an aluminum film, alumina vapor-deposited on thealuminum film, and a projective film formed on the alumina surface;

[0032]FIG. 4 is a schematic cross-sectional view of an essential portionof an optical scanning apparatus which represents a second embodiment ofthe present invention;

[0033]FIGS. 5A and 5B are diagrams showing the spectral reflectance of amirror having an aluminum film, and a dielectric film formed byvapor-deposition on the aluminum film;

[0034]FIG. 6 is a perspective view of an optical scanning apparatuswhich represents a third embodiment of the present invention;

[0035]FIG. 7 is a schematic cross-sectional view along a sub-scanningdirection of an essential portion of an image forming apparatus inaccordance with the present invention;

[0036]FIGS. 8A and 8B are diagrams showing the spectral reflectance of acopper mirror;

[0037]FIGS. 9A and 9B are diagrams showing the spectral reflectance ofcopper+alumina+SiO₂;

[0038]FIG. 10 is a schematic cross-sectional view of an essentialportion of an optical scanning apparatus which represents a fourthembodiment of the present invention;

[0039]FIG. 11 is a schematic cross-sectional view of an essentialportion of an optical scanning apparatus which represents a fifthembodiment of the present invention;

[0040]FIG. 12 is a schematic cross-sectional view of an essentialportion of an optical scanning apparatus which represents a sixthembodiment of the present invention;

[0041]FIG. 13 is a diagram showing the reflectance ratio and an anglecharacteristic of a bending mirror;

[0042]FIG. 14 is a diagram showing the reflectance-wavelengthcharacteristics of a bending mirror; and

[0043]FIG. 15 is a diagram showing transmittance-wavelengthcharacteristics of typical optical resins.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] Embodiments of the present invention will be described below withreference to the accompanying drawings.

First Embodiment

[0045]FIG. 1 is a schematic cross-sectional view of an essential portionof an optical scanning apparatus in which features of the presentinvention are best shown. Divergent rays of light from a semiconductorlaser 1, i.e., a light source, are converted into an approximatelyparallel light beam by a collimator lens 2 and the diameter of the lightflux is reduced by an aperture 3 to obtain a desired spot diameter. Thesemiconductor laser 1 used in this embodiment is a gallium nitridesemiconductor laser having an oscillation wavelength of 408 nm. A rotarypolygon mirror 5 is provided which reflects the light beam from thelight source so that the light beam travels to a surface 8 to bescanned. The reflected light beam from the polygon mirror 5 passesthrough scanning lenses 6 and 7 to form a small spot at any point in theentire scanning area. It is required that the scanning lenses 6 and 7have an fθ characteristic such that the light beam deflected at aconstant angular velocity by the polygon mirror 5 is converted into alight beam moving at a constant distance speed.

[0046] Each of the scanning lenses 6 and 7 used in this embodiment maybe made of either glass or a plastic. However, if the lens is made of aplastic, it is preferred that the lens satisfy conditions describedbelow with respect to a fourth or fifth embodiment of the presentinvention: Lmax−Lmin<3·log₁₀0.93/S, and S=log₁₀(1−3.55×10⁸/λ⁴), where λis the wavelength (nm) of the light beam.

[0047] It is also preferred that the lens alternatively satisfy thecondition Lmax−Lmin<10.0 (mm).

[0048] Also, at least one of a bending mirror, a. filter and an opticalthin film vapor-deposited on an optical member, each of which is amember for correcting light quantity distribution nonuniformity in asixth embodiment of the present invention, may be used in thisembodiment.

[0049] Further, an optical member in a seventh embodiment having acharacteristic (e.g., reflectance b of a bending mirror) similar to theinverse of the transmittance spectral characteristic of the opticalresin may be used. This optical member having a characteristic similarto the inverse of the transmittance spectral characteristic of theoptical resin may be a bending mirror, a filter, or an optical thin filmvapor-deposited on an optical member.

[0050] The parallel light beam is temporarily condensed on the polygonmirror 5 along a sub-scanning direction by a cylindrical lens 4, and thepolygon mirror 5 and the surface 8 to be scanned are set in an opticallyconjugate relationship with each other, thereby enabling correction ofsurface tilt of the polygon mirror 5.

[0051] The relationship between the complex refractive index and thereflectance of a metallic-film reflecting mirror will be described. Ifthe complex refractive index N of a metallic film is defined as

N(λ)=n(λ)−ik(λ)

[0052] where n, k>0;

[0053] n(λ) is the real part of the complex refractive index;

i={square root}{square root over (−1)};

[0054] k(λ) is the imaginary part of the complex refractive index(exhaustion factor); and

[0055] λ is the wavelength, then the reflectance R can be expressed as

R={n ₀ −n)² +k ²}/{(n ₀ +n)² +k ²}

[0056] where n₀ is the refractive index of the incidence medium,ordinarily n₀=1.0.

[0057] This equation is expanded into

R=1−4n/(k ² +n ²+2n+1)

[0058] and, when the lower limit of the reflectance of the metallicreflecting mirror used in the optical scanning apparatus is set to 0.8,

1−4n/(k ² +n ²+2n+1)>0.8

k ² >−n ²+18n−1

k>{square root}{square root over ((−n ²+18n−1))}.

[0059] Table 1 shows the complex refractive indices of typical metallicfilms with “A” representing the right side of the above inequality.TABLE 1 Wave- Al Cu Au Ag Cr length n K A N K A n K A N k A N k A 4000.42 4.00 2.60 0.85 2.00 3.88 1.52 1.75 5.35 0.08 1.93 0.67 2.45 1.797.01 450 0.55 4.50 3.03 0.87 2.20 3.93 1.40 1.75 5.11 0.06 2.42 0.292.54 1.89 7.15 500 0.70 5.00 3.48 0.88 2.42 3.95 0.82 1.70 3.80 0.062.87 0.29 2.65 1.98 7.33 550 0.85 5.50 3.88 0.72 2.45 3.53 0.31 2.492.16 0.06 3.32 0.29 2.77 2.02 7.52 600 1.08 5.90 4.43 0.17 3.07 1.450.18 3.10 1.51 0.06 3.75 0.29 2.93 2.05 7.77 650 1.30 6.30 4.91 0.133.65 1.16 0.15 3.50 1.31 0.07 4.20 0.51 3.10 2.10 8.03 700 1.60 6.505.51 0.12 4.17 1.08 0.15 3.75 1.31 0.08 4.62 0.67 3.31 2.11 8.34 7501.82 6.90 5.92 0.12 4.62 1.08 — — — 0.08 5.05 0.67 — — — 800 1.90 7.006.07 0.12 5.07 1.08 — — — 0.09 5.45 0.79 — — —

[0060] It can be understood from Table 1 that, in the case of copper(Cu), k>A and the reflectance is 80% or higher when the wavelength is onthe longer wavelength side of 600 nm, and k<A and the reflectance islower than 80% when the wavelength is on the shorter wavelength side of600 nm. In the case of aluminum (Al) or silver (Ag), k>A when thewavelength is 400 to 800 nm. Therefore, even when a light source of 500nm or less is used, it is possible to obtain a sufficient quantity oflight by using aluminum or silver.

[0061] Table 2 shows the reflectance R of each metallic film withrespect to a light flux having wavelength λ and striking the filmperpendicularly to the plane of incidence. TABLE 2 Wavelength Al Cu AuAg Cr 400 0.91 0.54 0.35 0.93 0.35 450 0.90 0.58 0.37 0.97 0.37 500 0.900.63 0.47 0.97 0.39 550 0.90 0.68 0.84 0.98 0.39 600 0.89 0.94 0.93 0.980.40 650 0.88 0.96 0.96 0.99 0.42 700 0.87 0.97 0.96 0.99 0.43 750 0.870.98 — 0.99 — 800 0.87 0.98 — 0.99 —

[0062]FIGS. 2A and 2B show the reflectance R of aluminum (Al) film withrespect to S-polarized light, which is a wave component vibrating alonga direction perpendicular to the plane of incidence, and P-polarizedlight, which is a wave component vibrating along a direction parallel tothe plane of incidence, the light being incident at angles of 20°, 40°,and 60°.

[0063] An ordinary process in which an anodized film is formed on asurface of an aluminum base member will be described.

[0064] The polygon mirror used in this embodiment is formed by using abase member made of aluminum. Ten and several polygon blanks eachcorresponding to such a member are combined to form an anode, andundergo electrolysis in an electrolyte satisfying certain conditions(e.g., boric acid), the electrolysis being performed at a voltage of 30to 40 V for 5 to 10 seconds. An oxide film is thereby formed on theblank surfaces. This alumite film has improved adhesion and uniformity,and the formation of this film can be controlled by selectingelectrolytic conditions and a period of time. Therefore, the filmthickness can be controlled with facility. In this embodiment,electrolytic conditions are set such that the minimum of the anglecharacteristic is set in the vicinity of 408 nm. To further form adielectric film as a protective film on the anodized film, film formingmay be performed by well-known vapor deposition or dipping.

[0065] In this embodiment, aluminum is used not only to form the firstmetallic film layer on the polygon mirror 5 but also to form the basemember. Thus, it is possible to reduce the manufacturing cost byeliminating the need for the step of forming a metallic reflecting filmon the base member. It is also possible to satisfy the above-describedreflectance condition by using a silver film. However, it is desirableto use aluminum rather than silver since silver is high-priced and issusceptible to environmental degradation.

[0066] According to the present invention, however, the material of thebase member is not limited to metals including aluminum. An insulatingmaterial may be used if various characteristic requirements of thepolygon mirror are satisfied.

[0067]FIGS. 3A and 3B show reflection characteristics of a polygonmirror formed by vapor-depositing on an aluminum film a film of alumina(Al₂O₃) having the function of reducing angle dependence, and forming aprotective film (dielectric film) on the alumina film. Such a filmstructure is effective in reducing the angle dependence of the polygonmirror with respect to P-polarized light and S-polarized light and inimproving the durability.

Second Embodiment

[0068]FIG. 4 is a schematic cross-sectional view along a sub-scanningdirection of an essential portion of an optical scanning apparatus whichrepresents a second embodiment of the present invention. The incidenceoptical system (not shown) from the semiconductor laser to the polygonmirror is the same as that in the first embodiment. In many cases of useof optical scanning apparatuses in image forming apparatuses, thescanning light beam is bent in the sub-scanning direction for somereason relating to the layout of units of the image forming apparatus.In this embodiment, the light beam moved for scanning along a horizontaldirection passes through scanning lenses 6 and 7 and is perpendicularlybent one time by a bending mirror 9 to travel to a photosensitive drum10. Ordinarily, a mirror used like the bending mirror 9 has a metallicfilm vapor-deposited on a surface of a glass base member. In thisembodiment, aluminum is vapor-deposited as described above to achievethe effect of obtaining a sufficient quantity of light even when a lightsource of 500 nm or less is used.

[0069] It is effective to vapor-deposit a dielectric film on thealuminum film as a means for further increasing the reflectance. FIGS.5A and 5B show the spectral reflectance of a mirror having MgF₂ film andZrO₂ film vapor-deposited on aluminum film. The reflectance is increasedby several percent in comparison with that in the case of thesingle-aluminum-layer structure shown in FIGS. 2A and 2B. To form amirror having a higher reflectance, multiple layers may be provided byalternately forming a low-refractive index film and a high-refractiveindex film.

[0070] While use of an aluminum film in the embodiment has beendescribed, any other metallic film may be effectively used according tothe present invention if the film satisfies the condition k>{squareroot}{square root over ((−n²+18n −1))}.

[0071] Each of the scanning lenses 6 and 7 used in this embodiment maybe made of either glass or a plastic. However, if the lens is made of aplastic, it is preferred that the lens satisfy conditions describedbelow with respect to the fourth or fifth embodiment:Lmax−Lmin<3·log₁₀0.93/S, and S=log₁₀(1−3.55×10⁸/λ⁴) where λ is thewavelength (nm) of the light beam.

[0072] It is also preferred that the lens alternatively satisfy thecondition Lmax−Lmin<10.0 (mm).

[0073] Also, at least one of a bending mirror, a filter and an opticalthin film vapor-deposited on an optical member, each of which is amember for correcting light quantity distribution nonuniformity in thesixth embodiment, may be used in this embodiment.

[0074] Further, an optical member in the seventh embodiment having acharacteristic (e.g., reflectance b of a bending mirror) similar to theinverse of the transmittance spectral characteristic of the opticalresin may be used. This optical member having a characteristic similarto the inverse of the transmittance spectral characteristic of theoptical resin may be a bending mirror, a filter, or an optical thin filmvapor-deposited on an optical member.

Third Embodiment

[0075]FIG. 6 is a perspective view of an optical scanning apparatuswhich represents a third embodiment of the present invention. Thisembodiment differs from the first or second embodiment in that ascan-imaging mirror 11 is used instead of the scanning lens 7. Asscan-imaging mirror 11, a cylindrical mirror, a spherical mirror, or alens with a freely curved surface, recently put to use with improvementsin plastic forming techniques, may be used. The advantage of use of thescan-imaging mirror 11 in the scan-imaging system resides in its havingboth an imaging lens function and a bending mirror function. Therefore,it is possible to remove the bending mirror described with respect tothe second embodiment. That is, a reduction in manufacturing cost can beachieved by reducing the number of components.

[0076] The scanning lens 6 used in this embodiment may be made of eitherglass or a plastic. However, if the lens is made of a plastic, it ispreferred that the lens satisfy conditions described below with respectto the fourth or fifth embodiment: Lmax−Lmin<3·log₁₀0.93/S, andS=log₁₀(1−3.55×10⁸/λ⁴), where λ is the wavelength (nm) of the lightbeam.

[0077] It is also preferred that the lens alternatively satisfy thecondition Lmax−Lmin<10.0 (mm).

[0078] Also, at least one of a bending mirror, a filter and an opticalthin film vapor-deposited on an optical member, each of which is amember for correcting light quantity distribution nonuniformity in thesixth embodiment, may be used in this embodiment.

[0079] Further, an optical member in the seventh embodiment having acharacteristic (e.g., reflectance b of a bending mirror) similar to theinverse of the transmittance spectral characteristic of the opticalresin may be used. This optical member having a characteristic similarto the inverse of the transmittance spectral characteristic of theoptical resin may be a bending mirror, a filter, or an optical thin filmvapor-deposited on an optical member.

[0080] Also in this embodiment, an aluminum film is used on thescan-imaging mirror to obtain a sufficient quantity of light even if alight source of 500 nm or less is used.

[0081] While use of an aluminum film in each embodiment has beendescribed, any other metallic film may be effectively used according tothe present invention if the film satisfies the condition k>{squareroot}{square root over ((−n²+18n−1))}.

[0082] From consideration of the reflectance characteristics shown inFIGS. 2A, 2B, 3A, 3B, 5A and 5B, it is said that the lower limit of thewavelength of the light sources used in the first to third embodimentsof the present invention is preferably 380 nm or greater.

[0083] The semiconductor laser 1 in the embodiments of the presentinvention may be of a multibeam type capable of producing two or morebeams.

Fourth Embodiment

[0084]FIG. 10 is a schematic cross-sectional view of an essentialportion of an optical scanning apparatus in which features of thepresent invention are best shown. Divergent rays of light from asemiconductor laser 1, i.e., a light source, are converted into anapproximately parallel beam by a collimator lens 2 and the diameter ofthe beam is reduced by an aperture 3 to obtain a desired spot diameter.The semiconductor laser 1 used in this embodiment is a gallium nitridesemiconductor laser having an oscillation wavelength of 408 nm. A rotarypolygon mirror 5 is provided which reflects the light beam from thelight source so that the light beam travels to a surface 8 to bescanned. The reflected light beam from the polygon mirror 5 passesthrough scanning lenses 6 and 7 to form a small light spot at any pointin the entire scanning area. It is required that the scanning lenses 6and 7 have an fθ characteristic such that the light beam deflected at aconstant angular velocity by the polygon mirror 5 is converted into alight beam moving at a constant distance speed. Further, the parallellight beam is temporarily condensed on the polygon mirror 5 along asub-scanning direction by a cylindrical lens 4, and the polygon mirror 5and the surface 8 to be scanned are set in an optically conjugaterelationship with each other at a sub-scanning cross section, therebyenabling correction of surface tilt of the polygon mirror 5.

[0085] The scanning lenses 6 and 7 used in this embodiment will bedescribed in detail. The scanning lens 6 is a glass lens made of a glassmaterial BSL-7 (a product from Ohara Inc.) and having antireflectioncoating films vapor-deposited on surfaces 6 a and 6 b through which thelight beam passes. The scanning lens 7 is a plastic lens formed byinjection molding of an optical resin ZEONEX480 (a product from ZEONCORPORATION).

[0086] The transmittance of an optical member is considered to beseparated into a surface reflection component P (reflection coefficient)and an internal transmittance τ.

Total transmittance T(λ)=P(λ)×τ(λ)   (1)

[0087] The reflection coefficient P depends on the refractive index n(λ)of the optical member and is expressed by the following equation:

Reflection coefficient P(λ)=2·n(λ)/(n(λ)²+1)   (2)

[0088] Also, the internal transmittance depends on the thickness t ofthe optical member and the following equation is established accordingto the Lambert's law.

Internal transmittance τ₂(λ)=τ₁(λ)^(t2/t1)   (3)

[0089] ZEONEX480 has a refractive index n(408 nm)=1.5402, and its totaltransmittance when the thickness is 3 mm is T₀(408 nm)=0.902 from thegraph of FIG. 7. From these values, the internal transmittance τ₀(408nm)=0.987 is obtained.

[0090] If the maximum ray passage distance of the plastic lens is Lmaxand the minimum ray passage distance is Lmin,

τ₁(408 nm)=τ₀(408 nm)^(Lmax/3)

τ₂(408 nm)=τ₀(408 nm)^(Lmin/3)

T₁/T₂=τ₁(408 nm)/τ₂(408 nm)=τ₀(408 nm) ^((Lmax−Lmin)/3)

[0091] Therefore, the transmittance ratio depends on the distancebetween the ray passage distances. In this embodiment, Lmax=7.50 (mm)and Lmin=3.21 (mm). Therefore, T₁/T₂=0.981, and it is possible to limitvariation in the quantity of light due to internal absorption to a smalllevel, 1.9%.

[0092] According to the result of a study made by the inventor, ifLmax−Lmin<3·log₁₀0.93/S, and S=log₁₀(1−3.55×10⁸/λ⁴), where λ is thewavelength (nm) of the light beam, it is possible to limit variation inthe quantity of light due to internal absorption in the plastic lens toa sufficiently small value in practice.

[0093] Further, according to the result of the study made by theinventor, if Lmax−Lmin<10.0 (mm), it is possible to limit variation inthe quantity of light due to internal absorption in the plastic lens toa sufficiently small value in practice.

Fifth Embodiment

[0094]FIG. 11 is a diagram showing the fifth embodiment of the presentinvention. The fifth embodiment differs from the fourth embodiment inthat two injection-molded plastic lenses. With the recent tendencies todevelop laser printers of lower prices, schemes have been put forth toreduce the manufacturing cost of laser scanner units. Plastic lenses canbe manufactured at a lower cost and can be formed with a surface in afreely curved shape, which cannot be attained in glass lenses.Therefore, plastic lenses also have an advantage over glass lenses interms of correction of aberrations.

[0095] In this embodiment, the shapes of the two plastic lenses areoptimized to achieve the effect of setting the polygon mirror and thesurface to be scanned in a conjugate relationship, the effect ofobtaining a scanning beam with an fθ characteristic and the effect ofsuitably correcting the curvature of field. However, since two plasticlenses are used, the total length of paths in the lenses through whichrays travel is increased relative to that in the case where one plasticlens is used as in the fourth embodiment.

[0096] In this embodiment, the thickness of each plastic lens is set soas to satisfy the above-described conditional inequality relating to thedifference between the ray passage distances. In this embodimentLmax=L₁₀+L₂₀ and Lmin=L₁₁+L₂₁. The maximum of the total ray passagedistance of the two plastic lenses (material: ZEONEX480) is Lmax=18.10(mm), the minimum of the total ray passage distance thereof isLmin=12.33 (mm), and T₁/T₂=0.976. Thus, variation in the quantity oflight due to internal absorption can be limited to a small value, i.e.,2.4%,. even though the two plastic lenses are used.

Sixth Embodiment

[0097]FIG. 12 is a schematic cross-sectional view showing a sub-scanningsystem of an optical scanning apparatus which represents a sixthembodiment of the present invention. The scanning optical system in thisembodiment uses two scanning lenses, and at least one of these lenses isa plastic lens. In this embodiment, by considering changes in anincident angle i of incidence on the bending mirror 9 with respect todifferent scanning angles, the optical system is designed so that if,for example, the ray passage distance of the plastic lens at a scanningcenter is longer than that at a scanning end (as in the fourthembodiment), the reflectance of the bending mirror decreases with theincrease in incident angle i (see FIG. 13), thereby reducing the totalamount of variation in distribution of the quantity of light on thescanned surface.

[0098] The ordinate of FIG. 13 represents the reflectance ratio (%),i.e., the value of a fraction in which the denominator is thereflectance at an incident angle of 60° while the numerator is anarbitrary incident angle.

[0099] As a correcting member for correcting light quantity distributionnonuniformity other than the bending mirror, a filter, an optical thinfilm vapor-deposited on an optical member, or the like may be used toachieve the above-described effect.

Seventh Embodiment

[0100]FIG. 14 is a graph showing spectral characteristics of an opticalresin and a mirror for explaining a seventh embodiment of the presentinvention. As mentioned above, in use of a semiconductor laser as alight source in a certain operational environment, variation inwavelength is inevitable because of a temperature-dependent oscillationwavelength characteristic of the laser. Therefore, it is required thatvariations in the optical characteristics, i.e., the transmittance, thereflectance, etc., of optical components used in the scanning opticalsystem be small in the vicinity of the laser oscillation wavelength.

[0101] A case where polycarbonate (PC) produced by Teijin Chemicals Ltd.is used will be described by way of example. A semiconductor laser usedas a light source is a gallium nitride semiconductor laser having anoscillation wavelength of 408 nm. Assuming that in an operationalenvironment there are wavelength variations of ±10 nm on 408 nm,variation in the quantity of light due to the optical resin is about1.0%, as represented by resin transmittance a shown in FIG. 14. However,it is possible to limit the total variation ratio c in the quantity oflight to approximately 1/20 of resin transmittance a by using an opticalmember having a characteristic (e.g., reflectance b of a bending mirror)similar to the inverse of the transmittance spectral characteristic ofthe optical resin. As an optical member having a characteristic similarto the inverse of the transmittance spectral characteristic of theoptical resin, in addition to a bending mirror, a filter, an opticalthin film vapor-deposited on an optical member, or the like may be usedto achieve the above-described effect.

[0102] The optical resin used in the fourth to seventh embodiments isonly an example of the resin material and the present invention is notlimited to particular optical resins. In other optical resins, thetransmittance may be reduced due to internal absorption with decreasingwavelength. By any application of the present invention to opticalsystems using such optical resins, the same effect can also be achievedaccording to the present invention.

[0103] In consideration of the reflectance characteristics shown inFIGS. 14 and 15, it is said that the lower limit of the wavelength ofthe light sources used in the fourth to seventh embodiments of thepresent invention is preferably 380 nm or greater.

[0104] The number of plastic lenses constituting each of the scanningoptical systems used in the fourth to seventh embodiments of the presentinvention may be three or more.

[0105] The semiconductor laser 1 serving as a light source, which isused in the fourth to seventh embodiments of the present invention maybe of a multibeam type capable of producing two or more beams. If thecombined system of the scanning optical system has a positive power ineach of the main scanning and sub-scanning directions (for imaging onthe surface to be scanned), the power of each single plastic lens may beeither positive or negative.

[0106]FIG. 7 is a schematic cross-sectional view along a sub-scanningdirection of an essential portion of an image forming apparatus 100using the optical scanning apparatus in accordance with the presentinvention. To the image forming apparatus 100, code data Dc from anexternal device 200 such as a personal computer is input. This code dataDc is converted into image data (dot data) Di by a printer controller121 in the image forming apparatus 100. This image data Di is input toan optical scanning unit 120 of the construction described above withrespect to one of the first to seventh embodiments. A light beam Lbmodulated according to image data Di is emitted from the opticalscanning unit 120. A photosensitive surface of a photosensitive drum 101is scanned with this light beam Lb in a main scanning direction.

[0107] The photosensitive drum 101, which is an electrostatic latentimage bearing member (photosensitive member) is rotated clockwise by amotor 105. With this rotation, the photosensitive surface of thephotosensitive drum 101 is moved relative to the light beam Lb along thesub-scanning direction perpendicular to the main scanning direction. Acharging roller 102 for uniformly charging the surface of thephotosensitive drum 101 is disposed above the photosensitive drum 101 soas to contact the surface of the same. The surface of the photosensitivedrum 101 charged by the charging roller 102 is irradiated with the lightbeam Lb moved for scanning by the optical scanning unit 120.

[0108] As mentioned above, the light beam Lb is modulated on the basisof image data Di. By irradiation with this light beam Lb, anelectrostatic latent image is formed on the surface of thephotosensitive drum 101. This electrostatic latent image is developed asa toner image by a development device 103 which is disposed on thedownstream side of the light beam Lb irradiation position in thedirection of rotation of the photosensitive drum 101 so as to contactthe photosensitive drum 101.

[0109] The toner image developed by the development device 103 istransferred onto a paper sheet 111 provided as a transfer member by atransfer roller 104 which is placed below the photosensitive drum 101 soas to face the same. Paper sheets 111 are accommodated in a papercassette 106 provided in front (on the right-hand side as viewed in FIG.7) of the photosensitive drum 101. Alternatively, paper sheet 111 may bemanually inserted therein. A sheet feed roller 107 is disposed at an endof the paper cassette 106 and is operated to feed each paper sheet 111from the paper cassette 106 into a conveyance path.

[0110] As described above, paper sheet 111 onto which the unfixed tonerimage has been transferred is further transported to a fixing deviceprovided at the rear (on the left-hand side as viewed in FIG. 7) of thephotosensitive drum 101. The fixing device is constituted by a fixingroller 108 having an internal fixing heater (not shown) and a pressureroller 109 placed so as to be maintained in pressure contact with thefixing roller 108. The unfixed toner image on the paper sheet 111transported from the transfer portion is fixed while being pressed andheated at the pressure contact portions of the fixing roller 108 and thepressure roller 109. Further, a sheet discharge roller 110 is providedat the rear of the fixing roller 108. The sheet discharge roller 110discharges the paper sheet 111 with the fixed image out of the imageforming apparatus 100.

[0111] Although not shown in FIG. 7, the printer controller 121 not onlyconverts data as described above but also controls, for example, themotor 105, internal components of the image forming apparatus 100, apolygon mirror motor in the optical scanning unit 120.

[0112] According to the present invention, as described above, in thescanning optical system in which the wavelength of the light source is500 nm or less and a metallic reflecting mirror is provided, a suitablemetallic material for the metallic reflecting mirror is selected toincrease the absolute reflectance and to improve the wavelengthdependent characteristic and the angle characteristic of thereflectance.

[0113] Thus, it is possible to obtain a sufficient quantity of lightrequired on the surface to be scanned without imposing an excessive loadon the semiconductor laser and without using an increased number ofcollimator lenses. It is also possible to reduce nonuniformity ofdistribution of the quantity of light on the surface to be scanned andto stabilize the quantity of light even when the wavelength of the lightsource changes in an operational environment.

[0114] According to the present invention, as described above, in thescanning optical system in which the wavelength of the light source is500 nm or less and at least one plastic lens is provided, a restrictionis put on the selection of the thickness of the plastic lens to reducenonuniformity of distribution of the quantity of light due to internalabsorption in the optical resin, thus guaranteeing image densityuniformity.

[0115] It is also possible to further reduce nonuniformity ofdistribution of the quantity of light on the surface to be scanned byplacing a light quantity distribution nonuniformity correction membersuch as a reflecting mirror.

[0116] Further, by providing an optical component having acharacteristic similar to the inverse of the wavelength characteristicof transmittance of the plastic lens, the stability of quantity of lightcan be improved even when the wavelength of the light source is changedin the operational environment.

1. to
 21. (Canceled).
 22. An optical scanning apparatus comprising adeflection optical system which deflects a light beam from a lightsource, and a scanning and imaging lens system which forms an imagingspot on a surface to be scanned with the light beam from said deflectionoptical system, wherein the wavelength of the light source is 500 nm orless, and wherein said scanning and imaging lens system has at least oneplastic lens, and an optical member having a spectral characteristicsimilar to the inverse of a wavelength characteristic of thetransmittance of said plastic lens.
 23. An optical scanning apparatusaccording to claim 22, wherein said optical member comprises areflecting mirror.
 24. An optical scanning apparatus according to claim22, wherein said optical member comprises a filter.
 25. An opticalscanning apparatus according to claim 22, wherein said optical membercomprises an optical thin film.
 26. An optical scanning apparatusaccording to any one of claims 22 to 25, wherein said light sourcecomprises a gallium nitride blue-violet semiconductor laser.
 27. Anoptical scanning apparatus according to claim 22, wherein said scanningand imaging lens system has at least one plastic lens; and if themaximum and the minimum of the total ray passage distance of saidplastic lens according to the deflection angle from the optical axis isLmax and Lmin, respectively, then Lmax−Lmin<10 mm is satisfied.
 28. Animage forming apparatus comprising an optical scanning apparatusaccording to any one of claims 22 to 25 or 27; a photosensitive memberdisposed at a surface to be scanned of said optical scanning apparatus;a development device which develops as a toner image an electrostaticlatent image formed on said photosensitive member by a beam of lightmoved in a scanning manner by said optical scanning apparatus; atransfer device which transfers the developed toner image onto atransfer member; and a fixation device which fixes the transferred tonerimage on the transfer member.
 29. to
 33. (Canceled).
 34. An opticalscanning apparatus according to claim 23, wherein said reflecting mirroris a bending mirror whose reflectance becomes higher as a wavelength ofthe light beam from the light source becomes shorter.